A Rare Encounter in the Thorax

Abstract Introduction Perivascular Epithelioid Cell (PEComa) tumors are rare mesenchymal neoplasm composed of smooth muscle cells and melanocytes. Given their scarcity, there is no clear diagnostic criteria; however, HMB45 positivity on immunohistochemistry is one common unifying feature. Here we present a case of an 80-year-old gentleman with a slow growing lung nodule that was found to be a biopsy proven PEComa. Case Presentation An 80-year-old male was referred to Interventional Pulmonology (IP) clinic for evaluation of a right lower lobe nodule that had increased in size from prior imaging. In 2014, the patient was found to have a 2-millimeter (mm) round, solid nodule on low-dose chest computerized tomography (CT). Repeat imaging between 2014 and 2024 showed a slow interval increase in size of the nodule to 9mm prompting his referral to IP. He reported no change in his respiratory symptoms during this period of time. A robotic assisted navigational bronchoscopy with transbronchial biopsies was performed. Surgical pathology demonstrated an eosinophilic neoplasm. Further analysis showed sections of cells with moderate amounts of eosinophilic granular and vacuolated cytoplasm and indistinct cell borders. There was mild nuclear pleomorphism, with smooth nuclear contours and occasional distinct nucleoli. Immunohistochemistry was positive for HMB45 and Melan-A. The nodule was classified as a PEComa. The case was presented at a multidisciplinary conference with the decision to surgical resection. The patient opted to defer treatment and continue yearly non-invasive monitoring of the tumor. Discussion Given the rarity of PEComas, there is a scarcity of information about the tumor. In our patient's case, the histologic morphology as well as the HMB45 and Melan-A positivity support the diagnosis of a PEComa. SF1 negativity on immunohistochemistry makes it unlikely to be of an adrenal primary origin, thus decreasing the likelihood of the nodule being consistent with an angiomyolipoma. The lack of cystic changes and difference in histologic features are inconsistent with lymphangioleiomyomatosis. The lack of high cellular glycogen on staining make a clear cell tumor the unlikely subtype. Our patient's nodule did not possess the characteristics consistent with a specific subtype leading to a diagnosis of an unspecified PEComa. The lack of diagnostic certainty makes treatment for patients with this diagnosis challenging and highlights the need for further investigation of this rare disease.